1. Field of the Invention
The present invention relates to a high-refractive index, low-dispersion optical glass, specifically an optical glass having a refractive index (nd) of 1.75 or more and an Abbe's number (νd) of 35 or more and to an optical element obtained by utilizing this optical glass, such as lenses or prisms. In particular, the invention relates to a high-refractive index, low-dispersion optical glass which is favorable for projection lenses or prisms of optical instruments represented by a camera or a projector required to have imaging properties with high precision and an optical element and an optical instrument prepared from that optical glass.
2. Description of the Related Art
In recent years, digitalization and high definition of optical instruments advance, and optical elements to be used for not only imaging instruments such as digital cameras or video cameras but also image reproduction (projection) instruments such as projectors or projection television sets are required to have high performance. In particular, a high-refractive index, low-dispersion glass is very high in demand as an optical element material such as various lenses, and the demand of an optical glass having a refractive index (nd) of 1.75 or more and an Abbe's number (νd) of 35 or more is especially large.
In addition, as to the performance required for the optical glass, not only characteristics including a refractive index, an Abbe's number and a degree of coloration but also the matter that a fluctuation in properties in the actual use environment is small are frequently required. This is because in the case where imaging properties largely change in the actual use environment, an optical element such as lenses or prisms is fixed by a tool in an optical instrument, and therefore, thermal expansion of the optical element is caused due to a change in temperature of the use environment (for example, a change in temperature in the inside of a casing, use at a high temperature, etc.), whereby a stress is generated in the optical element due to a difference in a coefficient of expansion from a fixing tool, and as a result, birefringence is generated in the optical element to cause a change in imaging properties.
Then, imaging properties designed on the basis of optical constants obtained under a fixed temperature condition (chiefly at around room temperature) such as a refractive index or an Abbe's number are not realized in the actual use environment. That is, the design must be made by supposing a use environment and expecting complicated fluctuations in properties at the time of optical design. This is not favorable from the standpoint of optical design.
Separately from the viewpoint on the optical design, in recent years, it is required more and more that an environmental load is small at the time of manufacturing of an optical glass or processing of an optical element.
Specifically, when an optical glass contains an environmentally harmful component such as lead (Pb) compounds or arsenic (As) compounds, a special measure is required for preventing the diffusion of a pollutant into air or water. Furthermore, the use of not harmful components but a large quantity of scarce mineral resources represented by tantalum (Ta) or the like involves disadvantages that not only are the production costs high, but also costs or labors for resource recovery are needed.
Optical glasses having a refractive index (nd) of 1.75 or more and an Abbe's number (νd) of 35 or more are disclosed in, for example, JP-A-2005-306732, JP-A-2002-284542, JP-A-2004-161506 and JP-A-2006-248897.
Though the glasses disclosed in JP-A-2005-306732, JP-A-2002-284542, JP-A-2004-161506 and JP-A-2006-248897 do not contain a lead (Pb) compound or an arsenic (As) compound in a glass composition thereof, all of these patent documents do not take into consideration a fluctuation in imaging properties in the actual use environment. Furthermore, JP-A-2002-284542 discloses a high-refractive index, low-dispersion glass with reduced tantalum (Ta). However, since this glass is required to contain a large quantity of ZnO, the fluctuation in imaging properties is large so that a disadvantage is easily generated from the standpoint of optical design. In addition, in recent years, as to ZnO, there is an environmental problem that influences against the water quality are a matter of concern.